Laminated glasses scatter fewer pieces of broken glass when they are damaged by external impact, and thus are excellently safe. Therefore, such laminated glasses are widely used in vehicles, railway carriages, aircrafts, ships, buildings, and the like. The laminated glass is produced by interposing an interlayer film between a pair of glass plates.
In order to reduce the weight of a laminated glass, studies have recently been performed for making a laminated glass thin. A thinner laminated glass, however, has a reduced sound-insulating property. If a laminated glass with a reduced sound-insulating property is used for the windshield of a vehicle, its sound-insulating property is disadvantageously insufficient against sounds at a register of about 5,000 Hz, such as wind noise and driving sound of wipers.
Then, additional studies have been performed for increasing the sound-insulating property of a laminated glass by changing materials of an interlayer film.
Patent Document 1 discloses, as one example of an interlayer film for laminated glass, a sound-insulating layer containing 100 parts by weight of a polyvinyl acetal resin with an acetalization degree of 60 to 85 mol %, 0.001 to 1.0 part by weight of at least one metal salt selected from alkali metal salts and alkaline earth metal salts, and 30 parts by weight or more of a plasticizer. This sound-insulating layer can be used alone as an interlayer film, or can be laminated with another layer and used as a multilayer interlayer film.